Radon is an invisible, odorless and chemically inactive radioactive gas that is produced by the decay of uranium ore, such as radium, actinium, or thorium. Radon undergoes a decay process which produces several high energy alpha particles and solid, radioactive decay products, termed daughter products. During the radon decay process, Rn222 decays by emitting an alpha particle having an energy of 5.5 MeV (million electron volts) to produce radioactive daughter Po218. Po218 then decays by emitting an alpha particle having an energy of 6.0 MeV to produce radioactive daughter Po214. Po214 subsequently decays by emitting an alpha particle having an energy of 7.7 MeV to produce radioactive daughter Po210. Po210 which has a half life of 20 years, eventually decays by emitting an alpha particle having an energy of 5.3 MeV.
The inhalation of radon and its radioactive decay products causes irradiation of lung tissue. Consequently, prolonged exposure to high concentrations of radon significantly increases the risk of developing cancer. It has been reported that the U.S. Environmental Protection Agency estimates exposure to naturally occurring radon leads to 21,000 lung cancer deaths nationwide each year, making radon the nation's primary environmental health threat and second only to cigarette smoking as a cause of fatal lung cancer.
Until recently, it was believed that dangerous concentrations of radon occurred primarily in uranium mines or laboratories containing large quantities of uranium. However, recent studies indicate that radon produced by the decay of radioactive ore in the rock and soil migrates to the earth's surface and becomes trapped in homes, where indoor concentrations of radon eventually build up to dangerous levels, thereby creating a significant residential health hazard. Indeed, indoor radon is now believed to be a greater radiological hazard to the population than all other natural and man-made radiation sources combined. It has been estimated that between 6 and 9 million homes in the United States have radon levels above 4 pCi/1 (pico Curies per liter of air), the level above which the Environmental Protection Agency urges remedial action.
The determination of whether a home is at risk due to unacceptably high concentrations of radon can be made only by actual measurement of air samples within the home. The tremendous volume of testing required to identify those homes which are at risk has created a need for a detector which possesses the following characteristics. First the measurement must be reasonably fast. One currently commercially marketed device, manufactured by Honeywell Inc., which relies on diffusion of ambient air past a radiation detection zone, suffers from a prolonged measurement period, requiring 24 to 48 hours to obtain a useful output. The instrument must also perform a high precision measurement. It should be relatively low cost in order to practically affordable and it should not require a skilled operator or the need for follow on laboratory analysis. In addition, the instrument should be capable of measuring radon concentration in pCi/1 and radon daughter product concentration in working level (WL) units.
Currently available radon detectors include scintillation and photomultiplier detectors, solid state junction and surface barrier photodiode detectors, gas proportional detectors, alpha track detectors, and charcoal canisters. However, none of these radon detectors has all of the above mentioned features. The above mentioned Honeywell radon detector, which at least uses a relatively simple and compact open photodiode detector to sense alpha particle emission, depends on the relatively slow diffusion of sampled air into the detector chamber, which makes its response time inordinately long (in the order of 24 to 48 hours).